Pete,
You can use a non-persistent object to ask the user for required parameters. The object should have an attribute with choices, which will be displayed as drop-down on the object form.
The query should look like this:
FIND MyObject WHERE MyObject.AnAttribute = QueryParameter.MyParameter AND <some-other-conditions>
You can then execute your query with a process like this:
ENTER NEW QueryParameter
DISPLAY MyQuery
QueryParameter will be discarded when the process finishes.
